This invention relates to a fuel assembly, and more particularly to a fuel assembly that is suitable for use in boiling water reactors
The upper structure of a conventional fuel assembly for use in boiling water reactors is constituted, as disclosed in Japanese Patent Laid-Open No. 58-113891, in such a manner that an upper end plug is provided at the top end of fuel rods, and an expansion spring is secured to each upper end plug for insertion into an upper tie plate. In the conventional example, even if a large amount of axial thermal expansion is generated in the fuel rod, the security of the fuel rod can be maintained by providing the expansion spring with a conical form.
However, such a fuel assembly is required to have its pressure loss reduced. Particularly in a case where the fuel assembly is mounted in the core of a boiling water reactor, the pressure loss is larger in the two-phase flow region (the upper portion of the fuel assembly) in which both liquid and vapor are present than in the single-phase flow region in which only the liquid in the fuel assembly is present.